Turbofan blades are typically provided with blade dovetail which are loosely mounted in complementary-shaped dovetail slots defined in the outer periphery of the rotor hub of the fan rotor. At engine operating speeds, the blades are urged firmly in position by the centrifugal force, thereby locking the blade dovetails against movement in the associated dovetail slots. However, when the fan rotates at low speeds, such as during windmilling, the centrifugal force is not sufficient to prevent the blade dovetails from moving in the dovetail slots. Windmilling may occur when wind blows through the engine of a parked aircraft causing the fan rotor to slowly rotate. Windmilling can also occur when an aircraft crew shutdown a malfunctioning or damaged engine in flight. The continued forward motion of the aircraft forces ambient air through the fan blades causing the fan rotor to rotate at low speed.
The opposing gravitational forces on the blade during such low speed rotation cause the blade to chafe against the disk due to the play at the joint between the disk and the blades. This low load high cycle event causes wear of the contacting surfaces. Such low speed rotation or windmilling induced wear can result in wear in critical stress locations and, thus, lead to premature retirement of blades and disk from service.
It is know, therefore, to provide an insert or spacer between the rotor disk and the blade root, to force the blade to its outward operating position, thus, reducing blade root movement during windmilling, and thus wear. Theses inserts are extra parts requiring extra time to make and install. They contribute to the overall complexity of the engine.
Accordingly, there is a need to provide a new and simple protection against windmilling induced wear.